Pulmonary embolism (PE) is a blockage or occlusion of a pulmonary blood vessel. Most commonly it is caused by a blood clot or thrombosis, that is, an “embolus”, in a vessel. It is a common illness with an annual incidence of 1 in a 1,000 in the Western world population. Mortality of PE is 30% when left undiagnosed and untreated but with treatment this can be reduced to 5-8%. The diagnosis of PE is difficult and is typically based on multi-step algorithms starting from an evaluation of the clinical probability and laboratory tests for markers, but positive diagnosis always requires some kind of imaging, ventilation-perfusion lung scintigraphy, pulmonary angiography, or multi-detector spiral X-ray computed tomography.
In an emergency department, the prevalence of PE is around 20%, which means that 5 patients are suspects for each case of actual PE. In these circumstances, the incidence of PE suspicion based on clinical probability can be estimated to be 1 in 200 of population. Diagnostic laboratory tests, called D-dimers, have good sensitivity to exclude PE but poor specificity to confirm it. Out of five suspected PE patients, only two are excluded with such diagnostic laboratory tests. Thus, of the remaining three patients, two will be PE negative and one will be an actual case of PE. Imaging these PE patients increases the cost of diagnosing PE. To reduce this cost, non-invasive diagnostic techniques with good sensitivity for excluding PE are needed.
To this end, indices derived from a comparison of expired breathing gas carbon dioxide (CO2) concentration with arterial blood CO2 partial pressure (PaCO2) have been experimented with. One such method plots expired CO2 over expired gas volume. The slope of the alveolar expiration portion of the plotted curve is then extrapolated to an expired gas volume comprising 15% of total lung capacity (TLC). The difference between the CO2 concentration determined by this extrapolation and the PaCO2 should be less than 12% of PaCO2 to exclude the existence of PE. This method suffers a weakness that reduces its diagnostic accuracy: that is, TLC values are statistical parameters determined from a large group of patients and expressed as nomograms for patient sex and size. PE suspected individuals may, however, differ a lot from these averages, which provides a source of error. In the worst case, this may result in false negative diagnosis of PE and a patient that is endangered with the high mortality of untreated PE.